1. Field of the Invention
The present invention relates to a vapor phase growth method of an oxide dielectric film, in particular, a vapor phase growth method of an oxide dielectric film having a perovskite crystal structure.
2. Description of the Prior Art
In recent years, ferroelectric memories (FeRAM) are studied which utilizes ferroelectric films as a capacitance insulating film. The ferroelectric memory is a nonvolatile memory utilizing the characteristics of a ferroelectric film whose polarization is maintained after voltage is applied. The properties required for the ferroelectric film include large residual polarization, a low polarization writing voltage, the uniformity of polarization properties in the element size of an order of 1 μm or less, and a low deposition temperature for achieving these properties.
Japanese Patent Laid-Open No. 2000-58525 describes the deposition method by CVD method of the perovskite ferroelectric film which is required to have these properties. The method includes first forming an early core or early layer in a first condition and then performing deposition by changing the conditions such as the supply quantity of a source gas containing a metalorganic material gas from the first condition to a second condition while maintaining the temperature. In accordance with the method, it is possible to deposit a perovskite ferroelectric film at a temperature of approximately 450° C. or less on an electrode of a metal such as Pt, Ru or Ir, or of a conductive oxide such as RuO2 or IrO2. Thus, it is also possible to form a ferroelectric film on the semiconductor substrate to which wiring of aluminum or the like is already formed.
In addition, as ferroelectrics have the anisotropy of polarization in the crystal axis direction, the control of orientation is needed for the uniformity of the polarization properties, and an attempt to control the orientation has been made by stacking atomic layers during the growth of a crystal. Such a method is indicated to have an effect to lower the temperature required for the crystallization. For example, for CVD method, “c-Axis-Oriented Pb(Zr,Ti) O3 Thin Films Prepared by Digital Metalorganic Chemical Vapor Deposition Method” by Y. Sotome et al. in Japanese Journal of Applied Physics Vol. 33 (1994) pp. 4066-4069 reports that c-axis-oriented lead zirconate titanate (Pb(Zr,Ti) O3, hereinafter referred to as “PZT”) can be prepared at 480° C. on a MgO single crystal substrate by repeating the operation of sequentially supplying a Ti source gas, a Pb source gas and a Zr source gas with amounts corresponding to the respective one atomic layer. In addition, “Orientation Control of Metalorganic Chemical Vapor Deposition-Bi4Ti3O12 Thin Film by Sequential Source Gas Supply Method” by T. Watanabe et al. in Japanese Journal of Applied Physics Vol. 39 (2000) pp. 5211-5216 reports that c-axis-oriented bismuth titanate (Bi4Ti3O12, BIT) can be prepared at 500° C. on a polycrystalline platinum electrode by controlling the order for supplying a Bi source and a Ti source to be supplied with amounts corresponding to one atomic layer.
Conventional methods however have disadvantages as described below. First, the method described in Japanese Patent Laid-Open No. 2000-58525 may produce defective cells due to the variations of the properties of capacitance elements, when memory cells having the capacitance elements of a minute size are highly integrated. This is because, as the size of the capacitance elements has become finer, the number of crystal grains in the ferroelectric film contained in one capacitance element is reduced, thereby enhancing the influence of one crystal grain having different properties.
On the other hand, the atomic layer deposition generally has a very low deposition speed. On the assumption that it takes five seconds to deposit one atomic layer, and about the same purging time is required between the deposition of atomic layers of different elements since oxide ferroelectrics include two elements or more as the, constitutional elements besides oxygen in almost all cases, about 20 seconds are required for preparing one crystal lattice with two elements. As 200 layers or more of unit lattices are required for preparing the ferroelectric having a film thickness of 100 nm, it takes 20×200=4000 seconds, that is, more than one hour. In addition, at a deposition temperature of approximately 500° C., migration between the deposited atomic layers due to the heat of substrates may occur thereby growing crystal grains larger, resulting in increased variations between elements as the size of the elements is reduced. By the same reason, solid phase reaction between the deposited atomic layers may easily occur thereby preventing sufficiently achieving the effect of the individual deposition of the atomic layers. Further, the CVD method does not necessarily produce the crystal growth surface parallel to the substrate, so that the effect for performing the atomic layer deposition cannot be sufficiently achieved.